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Computed Tomography
A single transmission measurement through the patient made by
a single detector at a given moment in time is called a ray.
A series of rays that pass through the patient at the same
orientation is called a projection or view.
Computed Tomography
Parallel beam & Fan
Beam geometry
All modern CT scanners incorporate fan beam geometry
Computed Tomography
The purpose of the CT scanner hardware is to acquire a large
number of transmission measurements through the patient at
different positions.
The acquisition of a single axial CT image may involve
approximately 800 rays taken at 1,000 different projection
angles, for a total of approximately 800,000 transmission
measurements.
Computed Tomography
Each ray that is acquired in CT is a transmission measurement through the patient along a line, where the detector measures an x-ray intensity It.
It and 1oare machine-dependent values
Introduction to CT Physics
µ reduces the dependency of the CT image on the machine-
dependent parameters , resulting in an image that depends
primarily on the patient's anatomic characteristics
Introduction to CT Physics
Do you know which re-construction method is used in CT scanner
to form an image?
Filtered back projection
Geometry and Historical Development of CT
CT scanners represent a marriage of diverse technologies, including computer hardware,motor control systems, x-ray detectors, sophisticated reconstruction algorithms,and x-ray tube/generator systems.
Geometry and Historical Development of CT
First Generation : Rotate Translate Pencil BeamSecond Generation: Rotate Translate Narrow Fan BeamThird Generation: Rotate Rotate Wide Fan BeamFourth Generation: Rotate StationaryFifth Generation: Stationary StationarySixth Generation: HelicalSeventh Generation: Multiple Detector Array
First Generation CT : Rotate Translate Pencil Beam
Only two x-ray detectors were used to measured the
transmission of x-rays through the patient for two different
slices.
This system used parallel ray geometry.
First Generation CT : Rotate Translate Pencil Beam
Starting at a particular angle, the x-ray tube and detector
system translated linearly across the field of view (FOV),
acquiring 160 parallel rays across a 24- cm FOV.
First Generation CT : Rotate Translate Pencil Beam
When the x-ray tube/detector system completed its
translation, the whole system was rotated slightly, and then
another translation was used to acquire the 160 rays in the
next projection.
First Generation CT : Rotate Translate Pencil Beam
This procedure was repeated until 180 projections were
acquired at 1 degree intervals.
A total of 180 x 160 = 28,800 rays were measured.
First Generation CT : Rotate Translate Pencil Beam
Pencil beam geometry makes inefficient use of x ray
Source but it provides excellent x-ray scatter rejection.
Do you know the advantage of First Generation CT Scanner?
First Generation CT : Rotate Translate Pencil Beam
As the system translated and measured rays from the
thickest part of the head to the area adjacent to the head, a
huge change in x-ray flux occurred.
The early detector systems could not accommodate this
large change in signal, and consequently the patient's head
was pressed into a flexible membrane surrounded by a
water bath.
First Generation CT : Rotate Translate Pencil Beam
The NaI detector also had a significant amount of
"afterglow”.
The signal from a measurement taken at one period of time
decayed slowly and carried over into the next measurement
if the measurements were made temporally too close
together.
Second Generation CT : Rotate Translate Narrow Fan Beam
Linear array of 30 detectors
Narrow fan angle of 10o
600 rays
540 views
324,000 data points
Second Generation CT : Rotate Translate Narrow Fan Beam
The shortest scan time with a second-generation scanner
was 18 seconds per slice, 15 times faster than with the first-
generation system.
Second Generation CT : Rotate Translate Narrow Fan Beam
Fan beam geometry makes use of a linear x-ray detector and
a divergent fan beam of x-rays.
X-rays that are scattered in the same plane as the detector
can be detected, but x-rays that are scattered out of plane
miss the linear detector array and are not detected.
Second Generation CT : Rotate Translate Narrow Fan Beam
Scattered radiation accounts for approximately 5% of the
signal in typical fan beam scanners.
Second Generation CT : Rotate Translate Narrow Fan Beam
Do you know disadvantage of narrow fan beam and several
detectors?
narrow fan beam allows more scattered radiation
to be detected than was the case with the pencil beam used in first-
generation CT.
Second Generation CT : Rotate Rotate Wide Fan Beam
The translational motion of first- and second-generation CT
scanners was a fundamental impediment to fast scanning.
At the end of each translation, the motion of the x-ray
tube/detector system had to be stopped, the whole system
rotated, and the translational motion restarted.
Third Generation CT : Rotate Rotate Wide Fan Beam
The number of detectors used in
third-generation scanners was
increased substantially (~ 800
detectors), and the angle of the fan
beam was increased.
Third Generation CT : Rotate Rotate Wide Fan Beam
The multiple detectors in the detector array capture the same
number of ray measurements in one instant as was required by
a complete translation.
The detector array formed an arc wide enough to allow the x-
ray beam to scan the entire patient.
Third Generation CT : Rotate Rotate Wide Fan Beam
"rotate rotate," referring to the rotation of the x-ray tube and the rotation of the detector array.
Third Generation CT : Rotate Rotate Wide Fan Beam
Do you know disadvantage of narrow fan beam and several
detectors?
Ring Artifact
Fourth Generation CT : Rotate Rotate Wide Fan Beam
fourth-generation scanners were designed specifically to
address “Ring artifacts”.
It is never possible to have a large number of detectors in
perfect balance with each other, and this was especially true
25 years ago.
Fourth Generation CT : Rotate Rotate Wide Fan Beam
Each detector and its associated electronics has a certain
amount of drift, causing the signal levels from each detector
to shift over time.
Detectors toward the center of the detector array provide data
in the reconstructed image in a ring that is small in diameter
and more peripheral detectors contribute to larger diameter
rings.